The reflection corneal topography (commonly called as “Placido” one) is a technique for measuring shape and curvatures of the human eye which stems from the Javal and Schiotz opthalmometer of 1889, from the studies of 1896 by Gullstrand (see Gullstrand A.: “Photographic-opthalmometric and clinical Investigations of corneal refraction”, Am J Optom Arch Am Acad Optom 1966; 43: 143-214), from the disk with concentric rings of 1880 by the Portuguese ophthalmologist Antonio Placido and “cone” embodiments thereof, starting from the Dekking one (see Dekking H M: “Zur Photographie der Hornhautoberflaeche”, Graefes Arch Ophtalmol 1930; 124:708-30) up to the subsequent developments (such as those disclosed, e.g., in U.S. Pat. No. 3,598,478 to Townsley and U.S. Pat. No. 4,772,115 to Gersten). Thanks to its size and outer shape, such ring cone allows to get closer to the eye, covering with the reflections of its mires, arranged on an inner reflecting or back-illuminated surface, almost the whole cornea, even though at the cost of a higher position sensitivity. In this regard, U.S. Pat. No. 5,526,073 to Mattioli comprises techniques for solving such criticality by acquiring the videokeratography (i.e. the photograph of the mires reflected on the cornea) at a precise distance from the corneal apex.
With reference to FIG. 1, the arrangement of the optical parts of a typical computerised reflection topography unit may be observed with a Placido back-illuminated mires cone. In particular, FIG. 1 schematically shows a Keratron® topography unit from Optikon 2000 S.p.A. company (that is simplified in the mires number), but it should be understood that other conventional reflection topography units, with Placido cone or disk having different shape, type, colour and arrangement of the mires, have in any case equivalent optical arrangements.
The mires cone 102 of transparent material back-illuminated by an illuminator 103, e.g. with surface mount LEDs, has mires marked on its inner surface (having shape of a frustum of cone in the embodiment of FIG. 1) which are constituted by axially symmetrical stripes alternatively black and white, starting from the central hole 108 of the cone 102 that appears black to the patient whose corneal topography of the eye 101 is measured. The image of these mires, reflected on the patient's eye 101 placed at reference distance from the cone 102, appears to the sensor 106 (e.g. a CCD or other sensitive element), through the lens 105, as a pattern of concentric alternatively black and white rings reflected on the cornea, for instance represented by the videokeratography 116 shown in FIG. 1-A (for the sake of simplicity, mires represented in the cone 102 are only a portion of those actually present, as shown by the image 116). Such image taken by the sensor 106, acquired and stored in an area of memory 109 of the computer with which the topography unit is provided, may be shown to the operator on a display. The computer may hence process the image 116 marking the mires and the pupil border, obtaining the image 117 shown in FIG. 1-B, and hence maps 118 representative of the same corneal geometry shown in FIG. 1-C. In this example the ectasia due to a keratoconus is shown by the left lower zone of the images 116 and 117, wherein the rings are close to each other, and of the image 118 where a high instant curvature, typically represented by an area 119 (conventionally in orange-red colours), is measured.
Mires may be arranged in a manner that may be defined as “homogeneous”, with a process graphically illustrated in FIG. 2, i.e. causing the image of the black-white and white-black borders to be angularly equally spaced in the photograph of the configuration (or pattern) reflected on a reference spherical surface (usually 43D, mean central curvature of a normal human eye, i.e. R=7.85 mm, since, according to the Javal convention, 1 Dioptre is equal to 337.5/R , where R is the axial or instant curvature radius in mm).
In other words, on the reference spherical surface, the radius of the image of the black-white border 111 of the first white mire, as well as the radial distance between such image and the image of the white-black border 112 of the second mire, between the latter and the image of the black-white border 113, and so on up to the distance between the image of the borders 114 and 115 (i.e. the thickness of the last reflected white ring), are all equal to each other.
FIG. 2 shows how mires (which in this example consist in 8 alternatively black-white white-black borders) may be marked in a homogeneous manner, after having determined: a) the number “n” of such mires; b) the angle “α” by which it is desired that each one of them is spaced from the preceding one in their image reflected on the reference sphere; c) the distance between the corneal apex 120 and the optical centre 121 of the lens 105 and d) the shape and position of the surface provided with such mires (preferably a conical frustum surface of revolution of which, in this example, we take the generatrix 122 as reference). In the sectional view of FIG. 2, the “reflected rays” from the sphere are marked backwards, starting from the optical centre 121 of the lens 105, at angles multiple of “α” with respect to the cone axis, until the same reference sphere is intersected. Hence, still backwards, the respective “impinging ray”, symmetrical to the reflected one with respect to the sphere perpendicular radius through the same point, is marked from each one of such intersection points. The “impinging rays” have increasing angles, respectively β0, β1, . . . βn, with respect to the cone axis. Finally the ideal positions of the mires are determined, from the intersection of said impinging rays with the generatrix 122 of the same surface.
However, it is actually possible to choose the angle “α” and the conical surface generatrix in such a manner that the position of the last mire “n” corresponds to a cone mouth of predetermined diameter and at predetermined distance from the eye.
The advantages deriving from a disposition homogeneous arrangement of this kind are known (see Mattioli R. & Tripoli N.: “Corneal Geometry Reconstruction with the Keratron Videokeratographer”, Optom Vis Sci 1997, 74, 881-894), especially if it is associated with a reconstruction of the corneal geometry with “arc-step” algorithms (see Klein SA.: “A corneal topography algorithm that produces continuous curvature”, Optom Vis Sci 1002; 69: 829-834; Tripoli N K, Cohen K L, Holmgren D E, Coggins J M: “Assessment of radial aspheres by the Keratron keratoscope using an arc-step algorithm” Am J Opthalmol 1995; 120: 658-664) which analyse the whole sequence of mires starting from the apex, with respect to “spherical approximation” algorithms which analyse them one by one (Cohen K L, Tripoli N K, Holmgren D E, Coggins J M: “Assessment of the power and height of radial aspheres reported by a computer-assisted keratoscope” Am J Opthalmol 1995; 119: 723-732). Actually, it is not mandatory that the arrangement is strictly homogeneous, e.g. many topography units adopt a central hole 108 wider than the just described one, but this reduces the central space resolution of the map processing and the whole precision.
Instead of the alternatively black-white and white-black circular borders, thin luminous or dark lines may be also used as mires, as commonly adopted by several topography units. However, under equal number of mires of this type the measurement of the maximum curvature that the sensor 106 is capable to detect before they mingle with each other is about half of that obtainable with alternating borders.
As shown in FIG. 1, a mirror 107 with a partially reflecting face, or a “beam-splitter” cube, reflecting the image produced by the assembly 104 for fixation of the patient, may be placed along the optical path between the hole 108 and the lens 105. Such image is generally a luminous point or picture focused at long distance by an optical system not shown in FIG. 1.
A need currently present in the field of measuring opthalmological is the possibility of combining other opthalmological measurements with the corneal topography. In particular, first of all among such measurements there is the ocular aberrometry (or wavefront analysis) having an increasing importance in the opthalmology field. Ocular aberrometry may be measured through several techniques: Laser ray-tracing (e.g. described in U.S. Pat. No. 6,409,345), Tscherning, SRR (Spatially Resolved Refractometer, e.g. described in U.S. Pat. No. 5,258,791), Talbot (e.g. described in U.S. Pat. No. 7,034,949) or the most diffused Shack-Hartmann or S/H technique (see Liang J, Grimm B, Goelz S, and Bille J F, “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor”, Journal of the Optical Society of America A 11, 1949-1957, 1994; Thibos L N, Faao P, Hong M Xin, “Clinical applications of the Shack-Hartmann aberrometer”, Optometry and Vision Science 76, 817-825, 1999). All these techniques, apart from SRR, substantially consist in projecting one or more laser beams into the eye and observing their projection on the retina, or collecting and projecting the emerging wavefront on a sensor measuring the distortions thereof. Radiation projected into the eye is in the near infrared, typically ranging from λ=780 and λ=940 nm, for both obtaining a good retinal reflex with low energy and not inducing involuntary pupil constriction, which would negatively affect such measurement.
Considering that the ocular aberrometry is often used in the same application context as the corneal topography, integrating corneal topography and aberrometry in only one instrument would offer the advantage of a better alignment between the two measurements. In particular, the photograph of the eye in the two conditions, under dilated pupil in aberrometry and under constricted pupil in photopic conditions, is extremely useful for centring the “eye-tracking” devices during, e.g., laser treatments. In clinical practice and in measurement for making customised contact lenses these complementary double information, topography and aberrometry, allows to foresee the ideal shape of a customised contact lens in both rear and front faces. Alignment and simultaneousity of the two measurements play a fundamental role even in the above.
Several methods have been adopted or proposed for combining reflection topography with an aberrometer.
In most cases, as for instance in the case of the equipments Nidek OPD-scan and Topcon KR-9000PW and of the U.S. Pat. No. 6,655,805 to Fujeda and U.S. Pat. No. 6,905,209 to Mihashi, the unavoidable compromise concerning the central hole of the Placido disk that has to be reasonably large and close to the eye for allowing aberrometry is accepted, compelling to give up a good coverage of the reflected mires within the central area of the topography.
If otherwise it is desired to keep a homogeneous arrangement of the mires as in topography units Keratron® the hole 108 would unavoidably limit the functions of aberrometry, or any other type of measurement combined with topography requiring a wide and close visibility of patient's eye.
By way of example, considering the topography units Keratron® with a 28 mires cone, cone mouth equal to 30 mm of diameter at a distance equal to 1 mm from the eye (i.e. from the reference position of the corneal apex), an eye-cone bottom distance equal to 63.13 mm, an eye-camera lens 105 distance equal to 100 mm and a homogeneous arrangement of the mires as described with reference to FIG. 2, the hole 108 has a diameter of 5.85 mm, definitely smaller than the pupil that is desired to measure with the aberrometry (up to 8 mm). It is possible to have a hole 108 slightly wider (keeping the homogeneity and the number of the mires) by increasing the cone depth, or vice versa, but this would further increase the distance of the aberrometer from the eye, thus reducing the related range of measurable emmetropia.
Other solutions have been proposed for combining in only one apparatus measurements of corneal topography and aberrometry. U.S. Pat. No. 6,234,631 to Sarver (and following ones to the same inventor) has proposed to employ multiple cameras, combining the aberrometry with the measurement of some corneal thicknesses in a reflection topography unit, wherein the Placido disk is made with a suitable chessboard pattern. However, such solution makes cornea reconstruction particularly complex and noisy and videokeratography not much familiar to the medical operator.
Other completely different solutions have been proposed in U.S. Pat. No. 6,050,687 to Bille and U.S. Pat. No. 6,634,752 to Curatu for combining corneal topography with aberrometry, but these, similarly to the solution proposed in U.S. Pat. No. 5,873,832 to Maloney, suffer from the drawback of being rather complex and are not an effective reflection topography since they provide the projection of a wavefront and/or of circular mires onto the eye through optical relays.
Another opthalmological measurement that could be profitably combined with reflection 1a reflection topography is the measurement of the corneal thicknesses with Scheimpflug camera, e.g. proposed in U.S. Pat. No. 5,341,180 to Isogai and U.S. Pat. No. 6,286,958 to Koest, wherein a thin slit light blade is projected perpendicularly to the cornea, at several axis angles, and the thus produced images in radial sections of the same cornea are observed by a camera in oblique position.
Other opthalmological measurements which would be advantageously combined with reflection topography are numerous. Such measurements comprise, by way of example and not by way of limitation, autorefractometry (e.g. proposed in U.S. Pat. No. 5,500,697 to Fujeda) and direct measurement of PSF (Point Spread function) of the eye (e.g. proposed in U.S. Pat. No. 6,273,566 to Kobayashi).